Track treads, and some 3d

Ok, I did a litte math to get an idea of how many treads I will need on the conveyor belts to make the tracks.

My drive sprocket is 8 tooth (thus, 8 valley’s or, the part between the teeth).   Diameter of the drive sprocket is 6″, so the circumference is:  18.84

So every 18.84 inches along the track, there needs to be 8 treads.

The track is 129″ long, so, to figure the number of treads per track, setup a little ratio/cross multiply:

8                    X
——–   = ———
18.84          129

X = 54 treads per track

So, I need 108 treads total.  not too bad I guess.   As long as making each one doesn’t require too many steps.

Ok, now for the fun stuff.  Finally did some 3d modeling in Blender3d (it’s a very slick, free program, look it up), as you can see from the new main page image.

I plan on building the body in 2 stages.  First, the tub, and rolling chasis.  Then a front nose piece and the rest of the body.  Something like this, do enjoy:

Scale: 1/4th

So I have finally determined the tank will be approximately 1/4 scale.   An M4 Sherman tank is a little over 19ft long, mine will be almost 5ft.. which is obviously about 1/4.

How I determined this?   I have 2 pieces of conveyor belt, 129″ long… one for each track.  My drive wheels and roller wheels are 5-1/8″.   I started drawing with a relative scale in MSPaint, decided to have the track come up at a 45° in the front and back, and the rest just sorta fell into place.   One cool part was determining the length of where the track wraps around the wheels in a circular fashion…. basically a trapezoid with rounded corners.

If you are into the math, it’s kinda cool, but also shows the different dimensions (note: length is not drawn to scale as it was the variable when I was figuring this out):

To be to the scale of a real Sherman, it would need to be only a little over 2 feet wide, but I will probably go a little bit wider, like 32-36″.  Now that I have some pretty decent dimensions figured out, I can start doing some 3d-modeling in Blender3d.   This will help a lot of my decisions down the road, such as building the lower pan, bogies, etc.

I am also currently looking at a few different options for the tracks treads.  The 2 finalists at this point are composite decking material, cut into slices on the table saw, or UHMV, a hard plastic, that is very versatile, and would hold up really well, but may be a little harder to come by at a reasonable price. Time will tell… but at least now I know what I’m building… a 1/4 Scale Sherman Tank.

Moron err, more on drive wheels…

One thing I glossed over was how I made my drive sprockets pattern.  I guess there are probably lots of ways to do this, but the way I did it is simple, and very accurate.  It’s main drawback being the number of teeth will always be 2^n, or(4, 8, 16, 32) … but, for me, an 8 tooth works good.

Here’s how I did it:

Looks like this:

If you read yesterdays post about Drive Wheels, but didn’t quite get how it all lines up, I made a cross section that may help to make sense of a few things… like how the belt sits on the plastic wheels, how I plan to connect the treads to the belt, and a sneak preview of how I plan on keeping the track from coming off … with the inside alignment blocks, which I think have a real name, but that’s what I call them.  So, without further ado, more MSPaint magic:

Drive Wheels from scratch

For my drive wheels, I want them to use some drive sprockets that sit on the outside of the conveyor belt (if you’re confused, read yesterdays post about Tracks).   But I figure why not also use some friction to my advantage, so I decided to have the conveyor belt roll along some rubber wheels (similar to those on the front of a lawn mower) , while at the same time have the drive sprockets push my overhanging treads.

I started with 2 floor flanges for 3/4″ pipe, and a 3″ length of threaded pipe:

hub

Then I use 4 bolts while I weld it to make sure everything’s nice and square:

After welding it up, I have some of the plastic/rubber wheels, drill aligning holes, and will eventually drill the same holes into the drive sprockets (the wood things).  These are 2 pieces of 5/8″ plywood glued together for now. They should hold up ok, if not, I will remake them out of something stronger later.  Here are a couple of pics of what it should look like when assembled (still need to finish the other drive sprocket on the bottom):

Finally, there will be a 5/8″ axle or long bolt going through the drive wheels.  This will either be a live half-axle, or fixed and the drive wheels will have their own bearings and have a pulley on the end.  I will deal with that later, much bigger fish to fry for the time being.

Type of TRACK to take tank through tough terrain.

Track type.   SO many types of tracks.  Snowmobiles, tanks, construction equipment vehicles.   Track design is crucial for the success of the type of vehicle.  For my application I need it to be reasonably light, and minimize friction.  I can’t have hundreds of separate metal plates hinged together.  And I don’t want to spend hundreds or thousands of dollars on rubber snowmobile tracks.

I have thus decided to make my own, custom.  I will use a 6″ wide, and about .25″ thick conveyor belt as the basis for my tracks.   Length will be discussed it great detail later.

I figure conveyor belts are designed to roll all day long, take tons of abuse, and most importantly have low resistance to rolling (less energy to operate).    Sounds perfect for not wasting too much juice from the batteries in my case.

Of course, there are 3 other elements to consider:  How will it get traction, be driven by the drive wheel, and stay straight on not come off.

Since the tracks will be driven by cogs on the outside of the tracks, I can solve traction and how to drive them by simply have a 1″x1″ treads that hang over the sides of the belt by about 1-2 inches on each side.   Like so:

basic track design

The basics of the outside of the track

As for keeping the track from coming off, that involves some teeth on the inside of the track, that fit between the other roller/guide wheels… which I will get to later.

For now, I need to decide what to use for the treads…. hmmm.   I’m thinking something like a hardwood, a composite like the decking material, some sort of plastic or hard rubber.  Not sure yet, but am keeping my eyes open for a material that fits the requirements (tough, drillable, won’t weather, not too abrasive, not too heavy, etc).

I will talk about my idea for the drive wheels next time.. have to finish those before I know the spacing of the treads on the tracks.

Hey, first blog

Welcome to my blog about building a small scale Tank .. yes, a two-tracked vehicle that can go over diverse terrain.  This will of course be driven by my son as soon as he’s old enough.  He is 3 months now, so I figured I have 2-3 years to finish it, of course much of that time will be used to test, change, test, repeat.  This blog will track my progress.

I’m choosing to use a track/drive system similar to a Sherman Tank.   It will be powered by two electric motors, one for each side, with independent forward and reverse.  I am still undecided on if it will be purely electric, or hybrid electric, which will be determined later as needed.

I am starting the blog today, but the planning of building the tank has already been in the works, and I have collected some crucial parts, which I will get into soon!

For now, I’ll leave you with a few teaser pics..   Till next time!

3d rendering done with Blender3d

Rendering of the drive system as currently envisioned